In the technique described in Japanese Patent Publication No. 6-6476, the pressurized-oil supply amount control device includes a main hydraulic pump 7 and a sub hydraulic pump 8 simultaneously driven by an engine 6, for example, as shown in FIG. 9. The pressurized-oil supply amount control device also includes a flow rate control valve 5 that controls the flow rate of pressurized oil discharged from the sub hydraulic pump 8. Thus, pressurized oil discharged from the main hydraulic pump 7 is merged with pressurized oil discharged from the sub hydraulic pump 8 and the flow rate of which is adjusted to any value by the flow rate control valve 5. The merged pressurized oil is then supplied to a control valve 3. The pressurized-oil supply amount control device includes an accelerator cylinder 4 and a governor 20 that controls the fuel injection amount of the engine 6. The accelerator cylinder 4 and the governor 20 are coupled together via a first link 21. Furthermore, the accelerator cylinder 4 and the flow rate control valve 5 of the sub hydraulic pump 8 are coupled together via a second link 22 operated simultaneously with the first link 21. Thus, the accelerator cylinder 4 and the flow rate control valve 5 are in a given operational relationship that allows the pressurized-oil supply amount to be reliably controlled.
In response to an operation input from a controller 120, the accelerator cylinder 4, which controls the rotation speed of the engine 6, is controlled. At the same time, the flow rate control valve 5 of the sub hydraulic pump 8, coupled to the accelerator cylinder 4, is operated via the second link 22. Thus, the pressurized oil discharged from the main hydraulic pump 7 is merged with the pressurized oil discharged from the sub hydraulic pump 8 and the flow rate of which is adjusted to a predetermined value by the flow rate control valve 5. The merged pressurized oil is then supplied to the control valve 3 in the crane.
The pressurized-oil supply amount control device described in Japanese Patent Publication No. 6-6476 enables possible noise from the engine to be inhibited and allows fuel consumption to be improved compared to, for example, a vehicle-mounted crane equipped only with a main hydraulic pump.
The displacement of the main hydraulic pump 7 is set such that the main hydraulic pump 7 can discharge pressurized oil of a rated pressure so as to prevent the engine from being stalled even in an idling condition in which the rotation speed and rotating torque of the engine are low. Furthermore, the displacement of the sub hydraulic pump 8 is set such that after the engine rotation speed and the rotating torque increase, the sub hydraulic pump 8 can be driven simultaneously with the main hydraulic pump 7 to discharge pressurized oil of the rated pressure.
The objective of this system is to, if the engine rotation speed and the rotating torque are low, return the pressurized oil from the sub hydraulic pump 8 to the tank 9 via the flow rate control valve 5, while supplying only the pressurized oil from the main hydraulic pump 7 to the control valve 3 side, in order to reduce a torque load on the engine 6. Furthermore, if the engine rotation speed and thus the rotating torque increase, the flow rate control valve 5, controlling the flow rate of the pressurized oil from the sub hydraulic pump 8, is opened. The pressurized oil from the main hydraulic pump 7 is thus merged with the pressurized oil from the sub hydraulic pump 8 to increase the pressurized-oil supply amount by a required value, while minimizing the engine rotation speed. Thus, required energy is saved, and possible noise is reduced.
Here, to achieve further energy saving and noise reduction in a pressurized oil supply amount control device, the pressurized oil from the sub hydraulic pump 8 may be merged while the engine rotation speed is lower. However, different types of vehicle on which the crane is mounted or different vehicle manufacturers use different engine rotation speeds to generate a rotating torque at which the main hydraulic pump 7 and the sub hydraulic pump 8 can be simultaneously driven to discharge pressurized oil of the rated pressure. Thus, the pressurized oil from the sub hydraulic pump 8 needs to be merged at the engine rotation speed depending on each vehicle.
However, in the technique described in Japanese Patent Publication No. 6-6476, the accelerator cylinder 4 and the governor 20 are coupled together via the first link 21, and the accelerator cylinder 4 and the flow rate control valve 5 are coupled together via the second link 22. Consequently, the relationship between the engine rotation speed and the control flow rate of the flow rate control valve 5 cannot be changed. Thus, to prevent the possible insufficiency of rotating torque of the engine even in a vehicle having an engine with different characteristics or the like, the device is set such that the engine rotation speed is increased to a slightly larger value to ensure the required rotating torque before the pressurized oil from the sub hydraulic pump 8 is merged. That is, the engine rotation speed increases almost in proportion to the travel distance of the link over the entire range of the travel distance. Thus, in order to always prevent the possible insufficiency of rotating torque of the engine, the device is set such that the engine rotation speed is increased to the slightly larger value to ensure the required rotating torque before the pressurized oil from the sub hydraulic pump 8 is merged.
Thus, according to the technique described in Japanese Patent Publication No. 6-6476, for example, in a vehicle that only needs a lower engine rotation speed to generate a rotating torque at which the main hydraulic pump 7 and the sub hydraulic pump 8 can be simultaneously driven to discharge pressurized oil of the rated pressure, the engine rotation speed may be increased more than necessary. Therefore, the technique described in Japanese Patent Publication No. 6-6476 still has room for improvement in terms of energy saving and noise reduction.